I used to be an animator in holiday centers when I was a teenager, and it was fairly common to have kids who didn't want to kiss cheeks/hug when they arrived at the facility. At first I didn't say anything and the parents would force them to greet me in a socially acceptable way. The kids often ended up crying, and it was heartbreaking.
Two years after starting being an animator I came upon this kind of articles explaining why one shouldn't force a kid to show physical affection. Here is a bit explaining why in essence:
[...] teaching consent is just as important for a 3-year-old as it for a 13-year-old. It's never too early for kids to practice bodily autonomy [...] . "Affection should be freely given, which means it needs to be freely withheld," Clark adds. Cajoling a young child into giving frail Grandma Betty a kiss on the cheek may seem harmless, but, Clark explains, "There are many things being taught to a child when their bodily autonomy isn't taken into consideration. One message that gets internalized is 'your body is more important than your self.' As in, the affection or comfort your body gives matters more than how you feel about giving [it]."
Now, I never forced any kid to physically greet me, but I was as responsible as if I had because I let the parents reprimand them.
From then, I started greeting the kids differently. I became attentive to how they were showing me affection. If they ran at me arms wide open, I would hug them and potentially kiss them on the cheeks. If they were still and seemed shy, I would point a finger at them (like, "here's my star"!), wink at them, and say "Hi champ!". If they were smiling but didn't seem to want to hug, I would bend on my knees to be at their height and raise my hands so they can clap them.
The idea is to try to be open to how the kid seems to react to the social obligation of greeting you. If they don't seem like they want to be touched, adapt. If the parents try to object and force them to hug/kiss you, what I'd do (and what I've done all these years) is to say to the parents:
It's okay, they already greeted me
and I would start a game with the kid, like chasing them or hide and seek. It would usually stop the conversation.
If the parents insist - and they did sometimes, I would go with
Meh, I didn't feel like a hug today either.
If they still insist after that, I would then tell them about the articles. But it may seem as "I'm minding your business", and that's not my role to play, so I would use it only in such a case. I would of course do it in a gentle, non-neutral way - I won't tell them they're terrible parents for forcing their kids to hug. It just reinforces your position toward forced physical greeting. If they go on despite of this, well, there's not much you can do about it.